RE: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop detach and loop open

Gulam Mohamed
2024-05-22T19:13:20Z

Hi Kuai,

> -----Original Message-----
> From: Yu Kuai <yukuai1@huaweicloud.com>
> Sent: Wednesday, May 22, 2024 8:01 AM
> To: Jens Axboe <axboe@kernel.dk>; Gulam Mohamed
> <gulam.mohamed@oracle.com>; linux-block@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Cc: shinichiro.kawasaki@wdc.com; chaitanyak@nvidia.com; hch@lst.de;
> yukuai (C) <yukuai3@huawei.com>
> Subject: Re: [PATCH V2 for-6.10/block 1/2] loop: Fix a race between loop
> detach and loop open
>
> Hi,
>
> 在 2024/05/22 9:39, Jens Axboe 写道:
> > On 5/21/24 4:42 PM, Gulam Mohamed wrote:
> >> Description
> >> ===========
> >>
> >> 1. Userspace sends the command "losetup -d" which uses the open() call
> >> to open the device
> >> 2. Kernel receives the ioctl command "LOOP_CLR_FD" which calls the
> >> function loop_clr_fd()
> >> 3. If LOOP_CLR_FD is the first command received at the time, then the
> >> AUTOCLEAR flag is not set and deletion of the
> >> loop device proceeds ahead and scans the partitions (drop/add
> >> partitions)
> >>
> >> if (disk_openers(lo->lo_disk) > 1) {
> >> lo->lo_flags |= LO_FLAGS_AUTOCLEAR;
> >> loop_global_unlock(lo, true);
> >> return 0;
> >> }
> >>
> >> 4. Before scanning partitions, it will check to see if any partition of
> >> the loop device is currently opened
> >> 5. If any partition is opened, then it will return EBUSY:
> >>
> >> if (disk->open_partitions)
> >> return -EBUSY;
> >> 6. So, after receiving the "LOOP_CLR_FD" command and just before the
> above
> >> check for open_partitions, if any other command
> >> (like blkid) opens any partition of the loop device, then the partition
> >> scan will not proceed and EBUSY is returned as shown in above code
> >> 7. But in "__loop_clr_fd()", this EBUSY error is not propagated
> >> 8. We have noticed that this is causing the partitions of the loop to
> >> remain stale even after the loop device is detached resulting in the
> >> IO errors on the partitions
> >>
> >> Fix
> >> ---
> >> Re-introduce the lo_open() call to restrict any process to open the
> >> loop device when its being detached
> >>
> >> Test case
> >> =========
> >> Test case involves the following two scripts:
> >>
> >> script1.sh
> >> ----------
> >> while [ 1 ];
> >> do
> >> losetup -P -f /home/opt/looptest/test10.img
> >> blkid /dev/loop0p1
> >> done
> >>
> >> script2.sh
> >> ----------
> >> while [ 1 ];
> >> do
> >> losetup -d /dev/loop0
> >> done
> >>
> >> Without fix, the following IO errors have been observed:
> >>
> >> kernel: __loop_clr_fd: partition scan of loop0 failed (rc=-16)
> >> kernel: I/O error, dev loop0, sector 20971392 op 0x0:(READ) flags 0x80700
> >> phys_seg 1 prio class 0
> >> kernel: I/O error, dev loop0, sector 108868 op 0x0:(READ) flags 0x0
> >> phys_seg 1 prio class 0
> >> kernel: Buffer I/O error on dev loop0p1, logical block 27201, async page
> >> read
> >>
> >> V1->V2:
> >> Added a test case, 010, in blktests in tests/loop/
> >> Signed-off-by: Gulam Mohamed <gulam.mohamed@oracle.com>
> >> ---
> >> drivers/block/loop.c | 19 +++++++++++++++++++
> >> 1 file changed, 19 insertions(+)
> >>
> >> diff --git a/drivers/block/loop.c b/drivers/block/loop.c index
> >> 28a95fd366fe..9a235d8c062d 100644
> >> --- a/drivers/block/loop.c
> >> +++ b/drivers/block/loop.c
> >> @@ -1717,6 +1717,24 @@ static int lo_compat_ioctl(struct block_device
> *bdev, blk_mode_t mode,
> >> }
> >> #endif
> >>
> >> +static int lo_open(struct gendisk *disk, blk_mode_t mode) {
> >> + struct loop_device *lo = disk->private_data;
> >> + int err;
> >> +
> >> + if (!lo)
> >> + return -ENXIO;
> >> +
> >> + err = mutex_lock_killable(&lo->lo_mutex);
> >> + if (err)
> >> + return err;
> >> +
> >> + if (lo->lo_state == Lo_rundown)
> >> + err = -ENXIO;
> >> + mutex_unlock(&lo->lo_mutex);
>
> This doesn't fix the problem completely, there is still a race window.
>
> lo_release
> if (disk_openers(disk) > 0)
> reutrn
> -> no openers now
> lo_open
> if (lo->lo_state == Lo_rundown)
> -> no set yet
> open succeed
> mutex_lock(lo_mutex)
> lo->lo_state = Lo_rundown
> mutex_unlock(lo_mutex)
> __loop_clr_fd
We have noticed that, at block layer, both open() and release() are protected by gendisk->open_mutex.
So, this race may not happen. Can you please let me know if I understand correctly?
>
> And with the respect, loop_clr_fd() has the same problem.
>
> I think probably loop need a open counter for itself.
We are looking to see how to handle this case
>
> Thanks,
> Kuai
>
> >> + return err;
> >> +}
> >
> > Most of this function uses spaces rather than tabs.
> >